1. Field of the Invention
The present invention relates to a voltage regulator that operates so that an output voltage may be kept constant.
2. Description of the Related Art
A conventional voltage regulator is described. FIG. 4 is a diagram illustrating the conventional voltage regulator.
As an output voltage VOUT increases, a divided voltage VFB of a voltage divider circuit 92 also increases. On this occasion, an amplifier 94 compares the divided voltage VFB with a reference voltage VREF, and accordingly when the divided voltage VFB becomes higher than the reference voltage VREF, a control signal VC also increases. Then, an ON-state resistance of an output transistor 91 increases to decrease the output voltage VOUT. As a result, the output voltage VOUT is kept constant.
On the other hand, as the output voltage VOUT decreases, the divided voltage VFB of the voltage divider circuit 92 also decreases. On this occasion, the amplifier 94 compares the divided voltage VFB with the reference voltage VREF, and accordingly when the divided voltage VFB becomes lower than the reference voltage VREF, the control signal VC also decreases. Then, the ON-state resistance of the output transistor 91 decreases to increase the output voltage VOUT. As a result, the output voltage VOUT is kept constant.
In this voltage regulator, it is assumed that the output voltage VOUT further decreases to be lower than a predetermined voltage value, that is, an undershoot has occurred in the output voltage VOUT. In this case, a current adder circuit 95 controls the amplifier 94 so that an operating current of the amplifier 94 may increase. Then, response characteristics of the amplifier 94 are improved to make rapid improvements to the undershoot, resulting in improved undershoot characteristics of the voltage regulator (see, for example, JP 2005-115659 A).
There may be a case where an output current limiting circuit is provided to serve as a protection function that is capable of limiting an output current so as to decrease the output voltage VOUT when the output current becomes an overcurrent.
In this case, in the conventional technology, even when the output current limiting circuit serving as the protection function has allowed the output voltage VOUT to decrease, an undershoot is determined to have occurred in the output voltage VOUT, and then the current adder circuit 95 causes the output voltage VOUT to increase. Therefore, there arises a problem that a circuit operation of the voltage regulator becomes unstable.